1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus. In particular, it relates to an electrophotographic image-forming apparatus of the type which is provided with a static eliminator disposed between an image-developing unit and an image-transferring unit for reducing the absolute value of the electric potential of the surface of a photosensitive component.
2. Description of the Related Art
Electrophotographic image-forming apparatuses come in several varieties, including laser printers, LED printers, photocopiers, etc. In such an image-forming apparatus, a light source, which consists of, e.g., an LED array, is provided for irradiating the surface of the photosensitive drum with light at a location between the image-developing unit and the image-transferring unit. Shining on the drum surface, the light serves to reduce the absolute value of the potential of the xe2x80x9cbackground portionxe2x80x9d (i.e., the portion which is not covered by toner) on the drum. In this manner, proper image-transferring performance and toner-cleaning performance can be expected.
A conventional image-forming apparatus of the above type has been found disadvantageous in the following point. The light source of a conventional apparatus is arranged to irradiate only a single region in the drum surface. In this manner, however, the absolute value of the potential on the drum surface is reduced sharply, for example, from xe2x88x92700V to 0V at one time. As a result, toner particles, which have once settled on the intended portion (xe2x80x9cimage portionxe2x80x9d) on the drum surface, may be scattered around due to the sudden potential drop and then stick to the background portion on the drum. This may degrade the print quality to an unacceptable extent.
More specifically, it is now supposed that the background portion and image portion on the surface of the photosensitive drum have a potential of xe2x88x92700V and 0V, respectively, as shown in FIG. 9. The difference in potential generates an electric field E between the background and the image portions. In general, an electric field between two points becomes greater as the potential difference between these points is greater and the distance between them is smaller. Due to the electric field E, a force F is exerted on the negatively charged toner particles, urging them from the background portion to the image portion. The electric field E is rendered greatest at points adjacent to the boundaries between the background portion and the image portion, since the distance between these points and the background portion is small. As a result, the toner particles located adjacent to the boundaries are most firmly fixed in position by the force F, so that they will not be unduly moved from the image portion to the background portion. This phenomenon is generally known as xe2x80x9cEdge Effect.xe2x80x9d
The illustrated potential difference (700V in FIG. 9) between the image portion and the background portion may be reduced to about 0V by irradiating the photosensitive drum with light. As a result, the electric field E (and hence the force F as well) will vanish.
Before the force F has gone, the toner particles are fixed in position on the drum surface, being pressed onto the drum surface by the force F which is balanced out by the reactional force Fr from the drum surface. However, when the force F suddenly disappears, the toner particles tend to be moved away from the drum surface by the reactional force Fr to be scattered around. In particular, the above-mentioned edge effect causes a greater amount of toner particles to collect at the boundaries between the image portion and the background portion. Thus, the scattering of toner particles by the reaction force Fr will occur to a greater extent at the boundaries.
Among the scattered toner particles, some may be attached to the background portion on the drum surface, as shown in FIG. 10. These unfavorable printing blots (hereinafter called xe2x80x9cprint dustxe2x80x9d) may unacceptably degrade the print quality.
It is, therefore, an object of the present invention to provide an image-forming apparatus which can enjoy excellent image-transfer and cleaning performances without suffering from the print dust as described above.
According to the present invention, there is provided an electrophotographic image-forming apparatus comprising: a photosensitive member having a surface upon which an electrostatic image is produced; a developing unit arranged adjacent to the photosensitive member; an image transfer unit arranged adjacent to the photosensitive member; and pre-transfer discharging means arranged adjacent to the photosensitive member between the developing unit and the image transfer unit. The pre-transfer discharging means is arranged to incrementally reduce an absolute value of an electric potential of a given point in said surface of the photosensitive member through more than one step as said given point is moved from the developing unit to the image transfer unit.
With such an arrangement, the absolute value of the electric potential of the surface of the photosensitive member is not sharply reduced between the developing unit and the image transfer unit. Thus, the toner-developed images produced on the surface of the photosensitive member are stably adhered to the surface, thereby causing no print dust.
In addition, the absolute value of the electric potential of the photosensitive member can be sufficiently reduced in the above manner while a given point in the surface of the photosensitive member is being moved from the developing unit and to the image transfer unit. Thus, the image transfer from the photosensitive member onto the recording medium and the cleaning of the surface of the photosensitive member can still be performed properly, as in the conventional apparatus.
According to a preferred embodiment, the pre-transfer discharging means is provided with a plurality of light sources for irradiating different portions of said surface of the photosensitive member.
Preferably, the photosensitive member may comprise a drum rotatable about an axis. In this instance, the light sources may be spaced from each other circumferentially of the drum.
According to another preferred embodiment of the present invention, the pre-transfer discharging means is provided with a single light source and a slit plate which is located between the photosensitive member and the light source.
Preferably, the slit plate may be formed with a plurality of slits for splitting light from the light source to irradiate different portions of said surface of the photosensitive member.
According to still another preferred embodiment of the present invention, the pre-transfer discharging means is provided with a single light source and a light controller for repeatedly turning on the light source at predetermined time intervals as said given point is moved from the developing unit to the image transfer unit.
Preferably, the image-forming apparatus of the present invention may further comprise a remnant charge discharger for irradiating the photosensitive member with light after a developed image is transferred onto recording medium. The pre-transfer discharging means is arranged to emit light whose specific light energy is no greater than one fifth (⅕) of a specific light energy of the light from the remnant charge discharger.
Preferably, the pre-transfer discharging means may be arranged to emit light of a predetermined wavelength which renders a spectral sensitivity of the photosensitive member no greater than half the maximum value of the spectral sensitivity.
Preferably, the pre-transfer discharging means incrementally may reduce the absolute value of the electric potential of said given point by no greater than 150V at one time.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.